If a time domain equalizer is used within a radio receiver such as a quadrature amplitude modulated (QAM) radio, an automatic gain control circuit of sorts is typically already incorporated within the time domain equalizer. While the prior art approach to automatic gain control for the overall circuit was to detect the signal, in the form supplied to the down converters and ultimately the time domain equalizer, and bandpass filter this signal, detect it, linearize it, low-pass filter it, threshold detect it, linearize it and supply the resultant signal back as a control voltage to a variable gain amplifier. The bandpass filter, detector, and detector linearizer can be eliminated by using the signal already existant in the time domain equalizer for both time domain equalization signal amplitude stabilization and for overall gain control of the traditionally used variable gain amplifier. This automatic gain control signal can either be from one of the automatic gain control circuits used for each of the quadrature phase signals or can be a combined signal derived from the in-phase and quadrature phase signals. In either event, the overall circuitry is considerably simplified and further, the more nearly constant amplitude signal appearing at the A to D converter reduces the possibility of errors and the overall circuit readily adapts to changing RF/baseband bandwidths.
It is therefore an object of the present invention to provide an improved automatic gain control circuit.